Automobile manufacturers are constantly seeking rugged, long-lived and efficient light sources to replace the conventional tungsten filament headlamps. Automobiles are harsh environments for any light source. The headlamps used by the current technology must usually be replaced several times over the life of an automobile. The typical tungsten halogen lamps in use today allow only about 1000 starts and about 2000 hours of lamp operation before burnout. Automobile manufacturers perceive a need for a lamp allowing 5000 starts and 5000 hours of operation without losing a significant portion of the lamp's initial light. A 15 percent drop in lamp intensity over the life of a lamp is generally considered satisfactory.
Automobile headlamps are necessarily positioned along the front surface of a vehicle. These surfaces are the first surfaces which encounter wind resistance as a vehicle moves. Lamp faces are, therefore, important to the aerodynamic design of the vehicle. The large lamp faces heretofore used had to be sculpted to fit in a vehicle's over all aerodynamic design. Consequently, this has led away from the standardization of headlamps. Limiting the lamp face size could lead back to standardization of headlamps, and decreased lamp costs.
Creating small, rugged, long-lived and inexpensive lamps for automotive service is not simple. In constructing automotive headlamps meeting these criteria, plastic has been employed for lenses and reflectors. Inexpensive and easily moldable, the use of plastic suffers, however, from the possibility of its being melted when overheated. It is, therefore, necessary to create lamps of high efficiency which are less prone to overheating at a given light output. A typical automotive headlamp requires between 50 and 60 watts of power to produce an output of 1100 to 1320 lumens with an efficacy of about 22 lumens per watt. There is a need to do much better.
HID lamps used in automotive applications have usually been the electroded type. These lamps are usually produced by press-sealing a glass envelope around a pair of electrodes. While the unmelted portions of the lamp envelope are accurately controlled in manufacture, the wall thickness, wall angles and press seal may vary from lamp to lamp. A small but still significant portion of the lamp's light passes through or is reflected from the press seal, particularly in small or short lamps where the seal area forms a greater percentage of the sphere of illumination. These variations may result in uncontrolled deflections of light, resulting in glare. The glass envelope could be controlled by exacting control of manufacturing details, but this would result in increased costs. There is, therefore, a need for an inexpensive HID lamp having accurately controlled wall thickness and wall angles.
Such a lamp is described in U.S. Pat. No. 5,113,121 (assigned to the same assignee as the present application and issued to Walter P. Lapatovich et al) for ELECTRODELESS HID LAMP WITH LAMP CAPSULE, on May 12, 1992. Described therein is an electrodeless lamp which eliminates many of the aforementioned problems caused by sealing a glass envelope around electrodes. The patent features the use of a high-frequency power source to energize the lamp.
Another desirable feature for lamps employed in automotive forward lighting (headlamps) is their ability to alter the beam pattern for blinking or flashing the headlamps under certain circumstances. Such blinking or flashing might, for example, be desirable for signaling traffic when a vehicle is accelerating into a passing lane and passing slower traffic. Such "flash-to-pass" signaling is required in certain countries. It is also desirable to switch the far-field illumination pattern of a headlamp from high beam to low beam when approaching oncoming traffic or in conditions of foggy or rainy weather.
Numerous methods for providing for high/low beam operation have been used in automobiles. To provide these functions, the traditional tungsten filament headlamps were often provided with an independent pair of filaments, designed to be energized either individually or collectively. Some automobile manufacturers used four, individual, single-filament, sealed beam headlamp units, two high beam (right- and left-side) and two low beam.
It is known in the art that the arc pattern in either electroded or electrodeless HID lamps exhibits acoustic resonance. At such acoustic resonance points, the arc is perturbed, such as, forced out of its normal physical pattern. Acoustic resonance can be induced by using an exciting signal, generally in the sub-audio, audio and supra-audio range, depending on the size of the lamp. It has been the practice of designers to avoid operating HID lamps at or near acoustic resonance points.
In U.S. Pat. No. 4,170,746 (issued to John M. Davenport on Oct. 9, 1979, for HIGH FREQUENCY OPERATION OF MINIATURE METAL VAPOR DISCHARGE LAMPS), the problems of avoiding acoustic resonance bands in the design of miniature HID lamps are discussed. It is well known in the art that, when electroded HID lamps operate at 60 Hz, their efficiency is relatively low. This efficiency problem may be overcome by using higher excitation frequencies, usually 20 kHz to 50 kHz. It is also well known that certain resonant frequencies exist where the arc pattern becomes erratic. Davenport identifies three different resonance bands. In the first band, catastrophic instability of the arc occurs; the arc is forced to the wall of the lamp and will quickly melt through the wall. A second resonance band exists where the light output fluctuates and the arc wanders. At a third resonance band, the luminous aureole surrounding the arc is unstable. Davenport teaches the design of electronic ballasts that are adapted to avoid these resonance bands.
U.S. Pat. No. 4,983,889 (issued to Victor D. Roberts on Jun. 8, 1991, for DISCHARGE LAMP USING ACOUSTIC RESONANT OSCILLATIONS TO ENSURE HIGH EFFICIENCY), teaches the use of acoustic energy to thoroughly mix the fill ingredients in the envelope of an HID lamp. Roberts discusses the effects of acoustic energy at both resonant and non-resonant frequencies on the geometry of the arc. Roberts, however, does not teach the use of acoustic energy to move the arc to accomplish aforementioned blink or flash operations.
It is an object of the present invention to provide a small, high-efficiency, electrodeless HID lamp that is suitable for use in automotive headlamp applications.
It is a further object of the invention to operate an HID electrodeless lamp at or near an acoustic resonance point, to deflect the HID lamp arc in a controlled manner to accomplish a visibly discernible change in the far-field illumination thereof, when associated with an optical system having forward gain.
It is yet a further object of the invention to achieve an acoustic perturbation of a transient nature to achieve flash-to-pass signaling or of a sustained nature to achieve high/low beam switching from a single electrodeless HID lamp.
It is a still further object of the invention to provide an electrodeless HID lamp that has a long service life and a low initial cost.